| 1 |
\section{Systematic uncertainties on the background prediction}
|
| 2 |
\label{sec:syst}
|
| 3 |
|
| 4 |
This section summarizes the systematic uncertainties on the predicted
|
| 5 |
backgrounds from \ttbar, both in the semileptonic and dileptonic decay
|
| 6 |
modes, and from rare processes.
|
| 7 |
For each signal region, the dominant \ttbar\ background component is normalized
|
| 8 |
to the sideband region, so the statistical
|
| 9 |
uncertainties in the data and MC yields in the sideband regions
|
| 10 |
contribute to the background predictions in the signal regions.
|
| 11 |
This normalization is repeated after varying the
|
| 12 |
rare background yield by $\pm50$\% to estimate the associated systematic uncertainty.
|
| 13 |
The uncertainty from the limited number of events in the MC samples
|
| 14 |
used to derive the predictions also contributes.
|
| 15 |
|
| 16 |
The dominant source of systematic uncertainty arises from the \mt\ and
|
| 17 |
\mbb\ extrapolation scale factors are derived in a sample
|
| 18 |
with (1 or )2 b-tags for the ($1$)$2\ell$ channels. The uncertainty is derived from control regions
|
| 19 |
where the MC prediction derived using the extraplation scale factor can be directly compared to data.
|
| 20 |
For the single lepton channel, the stability of the \mt\ scale factors to
|
| 21 |
\met\ resolution effects are investigated in the sample with 1 or 2
|
| 22 |
b-tags and by tightening the requirement on additional jets.
|
| 23 |
For the dilepton channel, the dependence of the \mbb\ scale factors on the
|
| 24 |
b-jet multiplicity is studied in the $1\ell$ \mt\ sideband region by
|
| 25 |
increasing the requirement on additional b-tags.
|
| 26 |
|
| 27 |
The stability of the background predictions is also verified by
|
| 28 |
comparing the prediction derived using the nominal \MADGRAPH sample
|
| 29 |
with those using samples generated with \POWHEG and \MCATNLO.
|
| 30 |
The resulting background predictions are consistent within the systematic
|
| 31 |
uncertainties, so no additional uncertainty is assessed from this test.
|
| 32 |
The systematic uncertainties are summarized in Table~\ref{tab:relunc_summary}.
|
| 33 |
|
| 34 |
\begin{table}[!h]
|
| 35 |
\begin{center}
|
| 36 |
\caption{Summary of the relative uncertainties (in \%) on the total
|
| 37 |
background predictions. All uncertainties are derived for the total
|
| 38 |
background prediction and are treated as uncorrelated from each other.}
|
| 39 |
\begin{tabular}{l||c|c|c|c}
|
| 40 |
\hline
|
| 41 |
Sample & 1l + 3b & 1l + $\ge$4b & 2l + 3b & 2l + $\ge$4b \\
|
| 42 |
\hline
|
| 43 |
\hline
|
| 44 |
Stat. & $9.3$ & $8.7$ & $20.8$ & $27.3$ \\
|
| 45 |
SF uncertainty & $7.8$ & $4.2$ & $10.0$ & $30.0$ \\
|
| 46 |
Top $\pt$ Modeling & $2.6$ & $0.6$ & $0.7$ & $4.7$ \\
|
| 47 |
rare bkg. Norm & $3.6$ & $0.8$ & $1.4$ & $5.6$ \\
|
| 48 |
Subdominant $\ttbar$ Norm & $1.0$ & $0.1$ & - & - \\
|
| 49 |
\hline
|
| 50 |
Total & $13.0$ & $9.7$ & $23.2$ & $41.2$ \\
|
| 51 |
\hline
|
| 52 |
\end{tabular}
|
| 53 |
\label{tab:relunc_summary}
|
| 54 |
\end{center}
|
| 55 |
\end{table}
|
